Why earthworms ?
Many varied interesting answers have already been posted. Most of them speak about nutritional concerns.
A point that I've not red about is :
I'v noticed that, during periods when I feed my axies, ribbed newts and other aquatics with earthworms only, the aquarium keeps clean. When I start giving pellets or pieces of fish flesh (because lack of earthworms), the tank become dirty more quickly (algae, trouble water and so on...).
I suppose earthworms are better assimilated by the caudates but I've no solid data about that.
Has anyone noticed something similar?
They certainly are a good choice for a food that doesn't foul the water